The present invention relates to a multi-cell filter apparatus of the type in which the filter cells are backwashed by a traveling backwash mechanism that progresses along the tank, for example as disclosed in U.S. Pat. Nos. 4,152,265 and 4,540,487, owned by the assignee of the present invention. In general, such filter apparatus include a filter bed supported above the bottom of the tank on a generally horizontal filter bed support with partitions provided at spaced locations along the tank and separating the bed into a plurality of filter cells each including an upper compartment above the filter bed support and a lower compartment below the filter bed support. A backwash carriage is mounted for movement along the tank in a carriage path and cell ports are uniformly spaced apart along a port locus paralleling the carriage path with each cell port communicating with the lower compartment of an associated one of the filter cells. A filtrate launder extends along the port locus and communicates with the cell ports to receive filtered water from the cells. The carriage is driven by carriage drive means along the carriage path in a backwash run.
A backwash head is carried by the carriage for movement along the port locus and backwash fluid is supplied to the backwash head during a backwash run to sequentially up-flow backwash the filter cells. The fluid used in backwashing the filter cells is normally filtered water and after completion of the backwashing of a filter cell, the filtrate compartment associated with that filter cell is usually filled with filtered water. When the filter cell that has been backwashed is returned to filter service, there is an initial period of relatively good effluent water from the filtrate compartment due to the clean backwash water remaining in the filtrate compartment. However, there then occurs a period in which the effluent quality from the backwashed filter cell is substantially poorer than the average effluent quality of the filter apparatus. The poorer quality effluent usually continues while the flow of material that remains in the bed and above the bed during backwashing, passes out of the backwashed filter cell and until a filter mat begins to form on the surface of the bed. The filter mat which forms on the surface of the bed during a filter run is much more effective in removing the suspended material in the effluent than the filter media alone, and the highest effluent quality is obtained from each cell after a filter mat has formed on the surface. Although the degraded effluent from the filter cell that has just been backwashed is mixed and diluted in the filtrate launder with the higher quality effluent from a number of other filter cells, this degraded effluent does affect the average effluent quality from the filter.
In the filter apparatus disclosed in U.S. Pat. No. 4,540,487, a traveling cell scavenging means was provided to downflow scavenge and rinse each cell after it has been backwashed, and before it was returned to service. The cell scavenging means included a fluid intake head mounted on the carriage at a location to trail the backwash head during movement along the backwash run and to operate when the carriage moved along the backwash run to sequentially withdraw fluid from each cell port and downflow scavenge the associated filter cell subsequent to the backwashing of each filter cell. U.S. Pat. No. 4,540,487 also discloses that the backwash pump can advantageously be arranged with its intake connected to the cell scavenging head so that fluid scavenged from the cells after backwashing was used in the backwashing of a succeeding cell.
The normal filtration rate in filters of the type described above is relatively low, generally of the order of two gallons per square foot per minute. However, a relatively high rate of flow, for example in the order of fifteen gallons per minute per square foot, is required for proper backwashing of the filter cells. In order to prevent break through of turbidity during scavenging, the downflow scavenging rate should only be slightly above the normal filtration rate, for example to about three gallons per square foot per minute.
Although the backwash rate is relatively high, the time required for backwashing is relatively low, for example of the order of one-half minute or less. However, since the scavenge rate must be relatively low to prevent break through, the time required for proper scavenging each cell would often be substantially greater than the time required for backwashing of a cell.
U.S. Pat. No. 5,089,117 discloses a filter apparatus in which clean water from a filtrate launder is first pumped through a backwash head in a direction to up-flow backwash a filter cell and then, while the backwash head remains in communication with that filter cell, fluid is pumped in the opposite direction from the backwash head in a downflow purging operation. In the embodiment of FIGS. 1 and 2 of that patent, the backwash head has a second port that trails the backwash port during a backwash run and fluid is pumped from the second port to a turbidity meter, to monitor the turbidity of that cell after backwashing. This arrangement requires that the backwash head dwell at each port for a time sufficient to first backwash the cell and thereafter purge the cell and significantly increases the time required for backwashing the filter apparatus. The embodiment disclosed in FIG. 3 of that patent differs from those shown in FIGS. 1 and 2 in not having any provision for purging filter beds following backwashing. The turbidity of the previously backwashed filter bed is monitored until the monitor signals that a predetermined level of turbidity exists in the liquid issuing from the previously backwashed filter bed. Since there is no purging of the filter cell prior to monitoring in the system disclosed in FIG. 3, the monitor may substantially delay advance of the backwash shoe to the next filter bed and correspondingly increase the amount of filtered water consumed during backwashing of the filter cell.